A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target on a substrate, e.g. a silicon wafer. The target portion may portion may include part of one or several dies. Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once. Conventional lithographic apparatus also include so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In the known lithographic apparatus movable objects, such as a movable substrate support and a movable patterning device support are used. These movable objects may be moved with high accuracy. To determine the position of the movable objects, position measurement systems capable of measuring a position with high accuracy have to be provided. For example, interferometer systems and encoder measurement systems have been provided for high accuracy position measurement of movable objects in a lithographic apparatus.
An encoder-type measurement system comprises at least one encoder-type sensor and an encoder scale, for example on a grid plate or other reference element. The encoder scale comprises an array of scale marks in at least a first direction. The at least one sensor is movable relatively to the encoder scale in at least the first direction and is configured to read the scale marks to determine a change in a relative position between the encoder scale and the at least one sensor in the first direction.
The encoder-type sensor may be mounted on the movable object, such as a movable substrate support or patterning device support, while the encoder scale is mounted or arranged on a reference object, such as a metrology frame. In an alternative embodiment, the encoder scale may be mounted on the movable object and the at least one encoder-type sensor may be arranged on a reference object.
The scale marks on the encoder scale may be optical marks, magnetic marks or other marks. The marks may be arranged at a periodic interval. The scale marks are readable by the encoder-type sensor such that by movement of the sensor relatively to the encoder scale a signal representative of relative movement between the encoder-type sensor and the encoder scale in the first direction may be obtained.
During manufacturing of the encoder scale, the scale marks are provided on the encoder scale. However, for example due to fabrication inaccuracies the scale marks may not be perfectly arranged on the encoder scale. Therefore, calibration of the encoder scale may be required. A first calibration may be performed by the manufacturer of the encoder scale after manufacturing. However, this calibration will not take into account scale errors introduced after the first calibration of the encoder scale.
For instance, during mounting of an encoder scale, for example by gluing, on a frame, errors may be introduced into the encoder scale. Also other causes such as damage of the encoder scale or contamination may result in errors in the encoder scale.